This invention relates generally to rapid setting cementitious compositions that can be used for a variety of applications in which rapid hardening and early strength is desirable. In particular, the invention relates to those cementitious compositions which can be used to make boards for use in wet locations in buildings, for example the Durock(copyright) board produced by the United States Gypsum Company. Such boards are made under conditions which provide a rapid setting of the cementitious mixture so that the boards can be handled soon after the cementitious mixture is poured into a stationary or moving form or over a continuously moving belt. Ideally, this will be as soon as 10 minutes, but more practically, setting of the cement mixture may be achieved up to about 20 minutes after being mixed with a suitable amount of water.
In U.S. Pat. No. 4,488,909, Galer et al discuss cementitious compositions capable of such rapid setting. Their compositions permit high speed production of carbon dioxide resistant cement boards by forming essentially all of the potential ettringite within about minutes after the composition is mixed with water. The essential components of their cementitious composition are portland cement, high alumina cement, calcium sulfate and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth and pumicite may be added up to about 25%. The cement composition includes about 14 to 21 wt % high alumina cement, which in combination with the other components makes possible the early formation of ettringite and other calcium aluminate hydrates, which are responsible for the early setting of the cementitious mixture.
In general, Galer""s formulation suffers from several major limitations. These limitations, as highlighted, as highlighted below, are even more of a concern for the production of cement boards:
The final setting times of the cementitious mixtures are typically greater than 9 minutes. The final setting time is defined further in the examples below, but more generally, the cementitious mixtures have set to the extent that cement boards can be handled and stacked, although chemical reactions may continue for extended periods.
The amount of high alumina cement in the reactive powder blend is very high. Typically, the high alumina cement is greater than 14 wt % of the reactive powder blend.
The amount of pozzolanic materials is limited to 25 wt % of the reactive powder blend.
Lime is required as an additional ingredient to obtain rapid set. Presence of excess lime in cement boards is detrimental to their long-term durability performance, because the cement boards of interest are reinforced with polymer coated glass fiber mesh that degrades, losing strength and ductility in a high alkaline environment. Presence of excess lime increases the alkalinity of the cementitious matrix and thereby negatively impacts the long-term durability performance of the polymer coated glass fiber mesh and the resulting cement board.
Ettringite is a compound of calcium aluminum sulfate compound having the formula:
Ca6Al2(SO4)3.32H2O
or alternatively:
3CaO.Al2O3.3CaSO4.32H2O
Ettringite forms as long needle-like crystals and provides rapid early strength to the cement boards, so that they can be handled soon after being poured into a mold or over a continuous casting and forming belt. In the Galer et al compositions, alumina required for the formation of ettringite is supplied by high alumina cement (HAC) that typically contains 36-42 wt % Al2O3. The bulk of the sulfate ions necessary for the formation of the ettringite are provided by adding gypsum, which is normally soluble in water. The lime needed for the formation of ettringite is provided by the portland cement and the added lime. The HAC is less soluble and is typically present in excess of that needed for ettringite formation. Thus, the gypsum and added lime are substantially consumed in the formation of ettringite. The availability of the HAC is usually increased by using a finely ground material. Since, an excess of HAC is present in the cement board, it would be desirable to reduce its consumption as HAC is one of the more expensive components.
High alumina cement has an advantage over portland cement since the HAC develops its maximum strength much earlier. However, it does not retain that strength over time as secondary reactions occur. It is of interest to note that the HAC does not set very rapidly by itself, it is its contribution to the formation of ettringite which makes it valuable in production of cement boards. Thus, the amount of HAC used in the cement boards should be limited to what is necessary for forming ettringite in quantities sufficient enough to allow handling of the cement boards. Also, since HAC is more expensive than portland cement, the cost of making the cement boards can be reduced substantially if HAC is limited to smaller amounts, or even eliminated.
In the Galer et al composition, about 72 to 80 wt % is portland cement, about 14 to 21 wt % is HAC, about 3.5 to 10 wt % is calcium sulfate, and about 0.4 to 0.7 wt % is hydrated lime. Other materials, such as pozzolanic materials, e.g. fly ash, superplastizers, and other cement additives may be included. The calcium sulfate was said to be in the form of gypsum (the dihydrate), the hemihydrate, anhydrite, or synthetic CaSO4. Since the gypsum was stated by Galer et al to be the most soluble component used in forming ettringite, and commercial grades of the dihydrate were preferred and exemplified in the ""909 patent, one skilled in the art would conclude that solubility of the gypsum source would be an important criteria in selecting the calcium sulfate source used in the cement boards. However, the present inventor has discovered that insoluble forms of calcium sulfate not only can be used in making cement boards, but that insoluble forms of anhydrous calcium sulfate (anhydrite) actually increase the speed with which ettringite is formed. This discovery makes possible a reduction in the amount of HAC in the cement board, while obtaining extremely rapid setting of the cementitious mixture. Alternatively the speed with which the cement boards are produced can be increased without increasing the amount of HAC used. Moreover, in the present invention it has also been discovered that very high proportions of pozzolanic materials such as fly ash may be used, with no externally added lime to obtain the rapid setting cementitious compositions of the invention.
It was an objective of the present inventor to develop a cementitious composition that is capable of developing an extemely rapid set while simultaneously satisfying the following conditions:
The reactive powder blend of the cementitious composition should contain low concentrations of high alumina cement. Reducing high alumina cement usage would help to lower the cost of the product since high alumina cement is the most expensive component of the cementitious composition. Also, decreasing the setting time could increase the production rate of cement boards.
The reactive powder blend of the cementitious composition should contain very high concentrations of pozzolanic materials (up to 55 wt % of the reactive powder blend) such as fly ash. Increasing use of pozzolanic materials such as fly ash would help to substantially lower the cost of the product. Moreover, use of pozzolanic materials would also help to increase the long-term durability of the product due to the pozzolanic effects.
The reactive powder blend of the cementitious composition should be free of externally added lime. Reduced lime content would help to lower the alkalinity of the cementitious matrix and thereby increase the long-term durability of the product.
The final setting time (i.e., the time after which cement boards can be handled) of the cementitious composition should preferably be between 3 to 10 minutes, and most preferably be between 3 to 6 minutes. A shorter setting time would help to increase the production output and lower the manufacturing cost of the product.
A rapid setting cementitious composition containing as reactive powders portland cement, pozzolan, high alumina cement, and insoluble calcium sulfate anhydrite, provides reduced setting times compared to the prior-art cementitious compositions. The composition preferably comprises as a reactive powder blend 35 to 90 wt % portland cement, 0 to 55 wt % pozzolan, 5 to 15 wt % high alumina cement, 1 to 8 wt % insoluble calcium sulfate anhydrite. Substitution of insoluble calcium sulfate anhydrite for conventional soluble gypsum (a dihydrate) increases the release of heat and decreases setting times, despite the use of very high amounts of pozzolanic materials, preferably fly ash. The cementitious composition may also include lightweight aggregates and fillers, plus additives to impart other useful properties as desired, such as superplasticizers, set retarders, and set accelerators.
In another aspect, the invention is a precast concrete product such as cement board made using the rapid setting cementitious composition described above.